This invention relates to imaging methods and systems. More particularly, the invention relates to a method and system for image reconstruction.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
One method of acquiring an NMR data set from which an image can be reconstructed is known as the “spin-warp” technique, which is a variant of the well known Fourier transform (FT) imaging technique. The spin-warp technique is discussed in an article entitled “Spin-Warp NMR Imaging and Applications to Human Whole-Body Imaging” by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (Gy) along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient (Gx) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT image acquisition, a series of pulse sequences is performed in which the magnitude of the phase encoding gradient pulse Gy in the pulse sequence is incremented (ΔGy). The resulting series of views that is acquired during the scan form an NMR image data set from which an image can be reconstructed. The acquisition of each phase encoded view requires a finite amount of time, and the more views that are required to obtain an image of the prescribed field of view (FOV) and spatial resolution, the longer the total scan time.
Many technical developments in the field of MR imaging aim to reduce data acquisition time. One such technical development is known as parallel imaging, in which multiple receive coils are used to reduce scan time.
In parallel imaging techniques, the reduction in scan time comes from omitting k-space lines or, equivalently, spacing the lines further apart. However, the reduction in k-space lines has the effect of reducing the FOV, thus producing image wrapping or aliasing if the object extends outside the reduced FOV in the phase encoding direction.
The aliasing associated with parallel imaging may be eliminated using one of two general techniques. In one technique, the missing k-space lines are synthesized using the receive coil sensitivity (B1 field) spatial information, followed by a Fourier transform to reconstruct an image without aliasing. This technique is called SiMultaneous Acquisition of Spatial Harmonics (SMASH), and is described by Sodickson, et al., “Simultaneous Acquisition Of Spatial harmonics (SMASH)”, Magnetic Resonance In Medicine 1997,38:591-603. In a second technique, the aliased images reconstructed from each coil are combined using the receive coil sensitivities to cancel the aliasing. This technique is called sensitivity encoding (SENSE) and is described by K. P. Pruessmann, et al., “SENSE: Sensitivity Encoding for Fast MRI”, Magnetic Resonance in Medicine 42, 952-962 (1999). Although these are the two most common parallel imaging methods, other generalized parallel imaging methods combine aspects of both techniques.
In the SENSE technique, the idea is to reduce acquisition time by increasing the step size (ΔGy) between phase encoding views, or equivalently, by reducing the FOV. In either case, the total number of views is reduced with a consequent reduction in scan time. If the object extends outside the reduced FOV, however, aliasing or wrap-around occurs in the phase encoding direction. The SENSE technique removes this aliasing by using knowledge of the surface coil receive field (also called sensitivities) to find the unaliased spin distribution. This knowledge is obtained by performing a sensitivity calibration scan prior to the reduced FOV (SENSE) scan. For example, the sensitivity calibration data may be obtained using combined signals from the surface coils as described in co-pending U.S. patent application Ser. No. 09/851,775 filed on May 9, 2001 and entitled “Calibration Method For Use With Sensitivity Encoding MRI Acquisition”.
During both the SENSE scan and the calibration scan, the patient may be required to hold his or her breath to minimize respiratory motion artifacts. If the patient and/or coil move in the phase encoding direction between the calibration and SENSE scans, for example if the patient were to intake a different volume of air during each of the scans, artifacts due to uncorrected aliasing can result.
One method of reducing artifacts due to patient motion between the calibration and SENSE scans is to extend the sensitivity measurement outside the boundaries of the object boundaries detected in the calibration scan using, for example, linear extrapolation. In conjunction with extending the sensitivity, the object boundaries are also extended to be outside those measured in the calibration scan. If the sensitivity and edges are extended sufficiently far, the sensitivity will be estimated at all pixels within the object at its position during the SENSE scan, and in addition, the number of aliased replicates will be greater than or equal to the actual number of replicates at all pixels. For typical scan prescriptions and typical respiratory motion, it has been found that edge and sensitivity extension by 10 to 20 pixels is sufficient. This enables accurate aliasing unwrapping at all pixels, even if the object moved after the calibration scan. However, the final image noise will be higher at pixels where the number of aliased replicates is overestimated. The increased noise frequently covers a large enough area to be noticeable and objectionable.
If the number of aliased replicates is overestimated, an adaptive, pixel-dependent regularization of the SENSE reconstruction has been used to reduce image noise. Adaptive regularization is a well-known numerical method used to reduce noise in the solution of inverse problems. However, one drawback to adaptive regularization is that it requires much iteration to determine the optimum regularization and therefore requires a long reconstruction time.
Another method of reducing artifacts due to patient motion between the calibration and SENSE scans is to combine both scans while the patient holds a single breath. Yet another method of reducing artifacts due to patient motion between the calibration and SENSE scans is to allow the patient to breathe freely while taking multiple averages during the calibration scan. These methods, however, increase the SENSE scan time, thus partially negating some of the advantages of the SENSE technique.